Typically hydraulic systems require filtration to preserve the cleanliness of the hydraulic fluid. Otherwise, components of the hydraulic system deteriorate and fail. During usage, filters scavenge foreign matter (dirt) and eventually begin to clog up. The pressure drop across the filter element for a given flow rate increases with the dirtiness of the filter element. Generally the filter is provided with a bypass valve that permits the bypass of the filter element when the pressure drop across the element reaches a desired limit. In this way, the hydraulic system can continue to function but, of course, unfiltered fluid is permitted to circulate. Since the filter is completely encapsulated, the filter element cannot be directly observed.
In the past, indicators have been provided to show when a bypass valve is open and/or to indicate a pressure drop across the filter. See U.S. Pat. No. 3,150,633. The prior art indicators do not indicate the filter condition but rather indicate the instantaneous pressure drop across the filter. The pressure drop is dependent upon the flow through the filter, the viscosity of the fluid and the cleanliness of the filter element. Since the flow in the hydraulic system is variable and the temperature of the fluid is variable (cold at start-up; warm after operation) the bypass first opens during surges at startup. Unless the pressure indicator is being observed at this time, the dirty condition of the filter and the fact that unfiltered fluid had been circulated is lost.
As with any apparatus, a dirty filter element indicator must have a minimum of parts. It must be inexpensive to manufacture. It must be rugged.
According to this invention, there is provided a filter condition indicator that latches the maximum pressure across the filter or latches the information that during surges unfiltered fluid has been circulated in the system. It is not necessary for an observer to position himself near the filter indicator during the start-up in order to know later whether or not the filter had been bypassed. It may even be dangerous to position oneself near the filter indicator while the equipment is being operated.